1. Technical Field of the Invention
The present invention relates generally to the construction of active matrix liquid crystal display (LCD) devices and, more particularly, to active matrix LCD devices integrally incorporating a peripheral drive circuit.
2. Description of the Prior Art
The prior art to which the invention is directed includes a conventionally known active matrix LCD device, in which a few hundred by a few hundred pixel electrodes are arranged in matrix form and thin-film transistors are arranged in individual pixels together with the pixel electrodes. In this kind of LCD device, the thin-film transistors located within the individual pixels control electric charges which are input to and output from the individual pixels.
The prior art also includes a recently developed construction known as a peripheral drive circuit integrated type. In this construction, both pixel matrix and peripheral drive circuit portions are integrally formed on a single glass substrate. The peripheral drive circuit integrated construction is advantageous for its reduced manufacturing costs and compactness of individual devices.
Generally, p- or n-channel thin-film transistors are arranged in a pixel matrix circuit as switching elements, while a peripheral drive circuit is a circuit constructed of p- and n-channel thin-film transistors.
In the manufacture of an active matrix LCD device of the aforementioned peripheral drive circuit integrated type, thin-film transistors to be arranged in its pixel matrix circuit and peripheral drive circuit are produced simultaneously on a single substrate.
The pixel matrix circuit and peripheral drive circuit generally require different transistor characteristics, however. For example, although thin-film transistors arranged in the pixel matrix circuit need not operate at so high speeds, they are required to be able to hold electric charges for the individual pixels, making it essential to have an extremely low off-state current. In contrast, what is most important for thin-film transistors arranged in the peripheral drive circuit are their high-speed operation and/or high-current drawing capabilities. Among them, thin-film transistors used to form a buffer circuit are required to have both high-speed operation and high-current drawing capabilities.
The thin-film transistors arranged in the peripheral drive circuit are also required to be less susceptible to degradation of their characteristics. This is because the higher the operating speed, the more likely to occur the degradation of characteristics.
The n-channel thin-film transistors have a mobility two to three times higher than the p-channel thin-film transistors, making it necessary to devise some provision for compensating for the difference in carrier mobility.